Sayyiduna Ibn Mas'ud (may Allah be pleased with him) used to say: "Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) was a strong fortress for Islam; people would enter Islam and not leave it. Then when Umar (may Allah be pleased with him) died, a breach appeared in the fortress, and now people leave it and do not enter it. Whenever he took a path, we found it easy. When the righteous are mentioned, then be sure to mention Umar (may Allah be pleased with him). He was the dividing line between increase and decrease (in Islam). By Allah, I wish that I could serve someone like him until I die."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8775